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By: Jen Smeage, Katie Johnson and Matt Floral
Published: 2011-09-20

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This study, conducted at ECHO during the Summer of 2011, was carried out to compare eggplant fruiting response to varying rates of a standard mineral fertilizer versus 6 month old compost with 25% cow manure and 75% plant matter (evenly split between woody and green, leafy material). The high rates were standard rates used on the ECHO Global Farm. Treatments included NPK (8%-2%-8%) fertilizer at 0%, 33%, 66% and 100% of the rate recommended by the University of Florida (200 lb N/acre), and compost at 33%, 66% and 100% of the ECHO-recommended high rate (1 liter/plant). The NPK fertilizer was applied in three equal amounts on 2 June (preplant), 13 July and 5 August, whereas all of the compost was applied on 2 June.  Seedlings (resulting from seeds sown in the greenhouse on 5 May  2011) of Solanum melongena ‘Florida Market’ were transplanted into raised beds on 2 June 2011. Fruit weights, averaged over all the rates, were twice as high with NPK fertilizer as compared with compost. The effect of fertilizer rate on yields was similar with NPK and compost. With both fertilizers, eggplant production increased with increased fertilizer rate. Results indicate that: (1) NPK achieved more production than compost; and (2) the full yield potential of eggplant was not reached with the highest  rates of either fertilizer. If conventional NPK fertilizer is to be replaced with compost, while still achieving the same level of production (especially on very sandy soils during an initial growing season), compost  will likely need to be applied at higher rates and/or combined with other practices (e.g. split applications and mulching). It is also recognized that soil biology takes time to develop, and the performance of compost may improve over subsequent growing seasons.